The invention is based on a speed control apparatus as generally defined hereinafter. For controlling the speed of vehicles, especially those equipped with internal combustion engines to drive them, it is known to dispose a train of circuit elements, amplifiers, comparators and final control elements, which replaces a mechanical linkage apparatus, between the first final control element subjected to arbitrary influence on the part of the vehicle driver (such as a gas pedal or the like) and the element which causes the engine to change its rpm or the torque which it puts out. Circuit systems of this kind, which are also known in the form of the so-called electronic gas pedal, known as "E-gas," have several advantages, among them that supplementary data can be incorporated into the engine control system, these data being evaluated such that unequivocally expressed wishes of the driver [regarding vehicle performance] can be put into practice at operating points of the engine having minimum fuel consumption. It is also possible for the transitions between the three ranges of acceleration, constant speed and deceleration to be embodied such that they can be adjusted smoothly and well. In addition to the advantages in terms of fuel consumption, the electronic gas pedal is also suitable for the larger classes of vehicles in particular, or in instances where a complicated guidance of the mechanical linkage from the gas pedal to a throttle valve in the intake tube of the engine, for instance, is required. Finally, it is possible to take into consideration additional functions such as those of a drive slippage and vehicle speed regulation system.
However, it could be considered disadvantageous in an electronic gas pedal regulating system that in the case of failure, for instance if the electrical power supply or some other system fails, then the components of the vehicle which participate in the control or regulation systems are virtually inoperable; also it must be expected that the electronic gas pedal control system and safety circuit will be associated with relatively high cost and will be complicated in structure.